Segmented chirped-pulse fourier transform spectroscopy

ABSTRACT

An emission can be obtained from a sample in response to excitation using a specified range of excitation frequencies. Such excitation can include generating a specified chirped waveform and a specified downconversion local oscillator (LO) frequency using a digital-to-analog converter (DAC), upconverting the chirped waveform via mixing the chirped waveform with a specified upconversion LO frequency, frequency multiplying the upconverted chirped waveform to provide a chirped excitation signal for exciting the sample, receiving an emission from sample, the emission elicited at least in part by the chirped excitation signal, and downconverting the received emission via mixing the received emission with a signal based on the specified downconversion LO signal to provide a downconverted emission signal within the bandwidth of an analog-to-digital converter (ADC). The specified chirped waveform can include a first chirped waveform during a first duration, and a second chirped waveform during a second duration.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.13/912,548, filed on Jun. 7, 2013, which is a continuation-in-part under35 U.S.C. 111(a) of International Application No. PCT/US2012/029430,filed Mar. 16, 2012 and published on Sep. 27, 2012 as WO 2012/129089 A1,which claims benefit of priority under 35 U.S.C. 119(e) to U.S.Provisional Patent Application Ser. No. 61/454,223, filed on Mar. 18,2011, the benefit of priority of each of which is claimed hereby, andeach of which are incorporated by reference herein in its entirety. U.S.patent application Ser. No. 13/912,548 also claims the benefit ofpriority under 35 U.S.C. 119(e) to U.S. Provisional Patent ApplicationSer. No. 61/656,665, filed on Jun. 7, 2012, which is hereby incorporatedby reference herein in its entirety.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No.CHE-0847919 awarded by the National Science Foundation (NSF). Thegovernment has certain rights in the invention.

BACKGROUND

Spectroscopy, such as rotational spectroscopy, is a powerful structuraltool in physical chemistry. For example, the relationship between amolecular structure and its rotational transition frequencies can beused for structure determination of gas phase samples. Other effects inthe rotational motion of molecules, such as centrifugal distortion,hyperfine spectral structure from quadrupolar nuclei, or frequencyshifts caused by tunneling motion, can be used to provide furthercharacterization of the molecular structure and low frequencyvibrational motions.

The millimeter (mm) wave or terahertz (THz) frequency region is aparticularly useful region for chemical detection and characterization.For room temperature samples, this is the region where pure rotationalspectra are most intense. All molecules with a permanent dipole momenthave a pure rotational spectrum. Such a spectrum includes a large numberof sharp transitions that can serve as a “fingerprint” of a molecularidentity. For example, such transitions can be about 1 megahertz (MHz)in width at about 300 gigahertz (GHz) in a low-pressure gas cell. In oneapproach, the transition frequencies can be related back to themolecular identity, such as through fitting with a molecularHamiltonian, which can explain measured transition frequencies andrelative intensities in terms of calculable physical parameters. Fromthe intensities of the transitions observed by millimeter- andsub-millimeter-wave spectroscopy, absolute molecular concentrations canbe derived with high accuracy and selectivity.

OVERVIEW

In one approach, techniques for millimeter-wave spectroscopy involvemeasuring molecular transitions based on their absorption of radiation,using a synthesizer in the microwave frequency region, coupled to one ormore frequency multipliers, as the radiation source. However, the slowscanning and switching speeds of such synthesizers preclude rapiddetection of large bandwidths. For example, monitoring of complexchemical mixtures at desired video refresh rates is not possible usingthese techniques.

The present inventors have recognized, among other things, that ahigh-speed Digital-to-Analog converter (DAC), such as a high-speedArbitrary Waveform Generator (AWG), can provide a frequency-agile sourceof energy including microwave frequencies, and can open newopportunities for the fast detection of millimeter wave spectra whenincluded in a spectroscopy system instead of or in addition togenerally-available synthesizers.

In an example, a DAC can create a high-bandwidth linear frequency sweep(e.g., a chirped pulse), which can be amplified such as to inducepolarizations in a molecular sample at the frequencies of transitionswithin the bandwidth of the pulse. The sample then continues to emitradiation at the frequencies of the transitions, and such free inductiondecay (FID) emission signals can be digitized and Fourier transformed toyield a molecular spectrum. Such Chirped-Pulse Fourier Transform (CP-FT)techniques generally provide high sensitivity, dynamic range, andfrequency accuracy in the detection of broadband molecular spectra. Dueto the fast acquisition speed, short-lived species generated in samplecells, such as ions and radicals, can be characterized using CP-FTtechniques. However, generally-available digitizer bandwidths arelimited. Accordingly, the present inventors have also recognized that tocollect higher-bandwidth spectra (e.g., spectra having a total bandwidthwider than an available digitizer bandwidth), a technique to reduce therequired digitizer bandwidth is needed.

The present inventors have recognized, among other things, that a“segmented” CP-FT technique can be used, such as using the DAC togenerate a chirped-pulse waveform, and using the DAC to generate one ormore Local Oscillator (LO) frequencies. Such a segmented CP-FT methodcan reduce the bandwidth demanded for the digitizer by a “divide andconquer” approach that segments the total measurement bandwidth into aseries of segments. For example, a bandwidth of a frequency multipliedchirped-pulse waveform can be specified at least in part based on anavailable digitizer bandwidth, and each segment can include a chirpedpulse specified to excite the sample in such a manner that resultingemissions can be captured within the available digitizer bandwidth. Atotal bandwidth of interest to be measured can be wider than thedigitizer bandwidth, and can be obtained via generating a series ofrespective chirped pulses and LO frequencies corresponding to eachsegment, obtaining emissions for each segment, then assembling anensemble of respective estimated spectra corresponding to each segmentto provide coverage of the total bandwidth of interest.

Using a DAC-based source, these segments can be measured in rapidsuccession to cover large frequency ranges. For example, receivedemissions can be downconverted using a heterodyne detection technique.Such heterodyne detection generally uses a high agility,phase-reproducible local oscillator (LO) source and this source can alsobe DAC-based, and is thus very rapidly tunable.

In an example, an emission can be obtained from a sample in response toexcitation using a specified range of excitation frequencies. Suchexcitation can include generating a specified chirped waveform and aspecified downconversion local oscillator (LO) frequency using adigital-to-analog converter (DAC), upconverting the chirped waveform viamixing the chirped waveform with a specified upconversion LO frequency,frequency multiplying the upconverted chirped waveform to provide achirped excitation signal for exciting the sample, receiving an emissionfrom sample, the emission elicited at least in part by the chirpedexcitation signal, and downconverting the received emission via mixingthe received emission with a signal based on the specifieddownconversion LO signal to provide a downconverted emission signalwithin the bandwidth of an analog-to-digital converter (ADC). Thespecified chirped waveform can include a first chirped waveform during afirst duration, and a second chirped waveform during a second duration.

The spectroscopy techniques and apparatus described herein can begenerally applicable to both the microwave frequency region, and muchhigher frequencies (e.g., mm- or sub-millimeter wavelengths—whichinclude frequencies in the terahertz (THz) region of the electromagneticspectrum). For example, generally-available frequency multipliers canprovide signals into the mm-region (or even shorter wavelengths), andhave made possible the generation of phase-stable radiation with highfrequency precision for high-sensitivity spectroscopy.

This overview is intended to provide an overview of subject matter ofthe present patent application. It is not intended to provide anexclusive or exhaustive explanation of the invention. The detaileddescription is included to provide further information about the presentpatent application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates generally an example of a system for generating achirped-pulse excitation signal, and for obtaining emission from asample in response to such excitation.

FIG. 2A illustrates generally an illustrative example of a mm-wavespectrometer system, such as including a sub-harmonic mixer having twoinput ports.

FIG. 2B illustrates generally an illustrative example of a mm-wavespectrometer system, such as including a fundamental-mode mixer havingtwo input ports.

FIG. 2C illustrates generally an illustrative example of a mm-wavespectrometer system, such as including a sub-harmonic mixer and asingle-channel arbitrary waveform generator (AWG) implementation.

FIG. 3A illustrates generally an illustrative example of a spectrogramrepresentation of the DAC channel outputs, such as corresponding to adual-channel example, and FIG. 3B illustrates generally a detailed viewof a portion of FIG. 3A.

FIG. 4A illustrates generally an illustrative example of a time-domainwaveform including a chirped pulse followed by a single-frequency LOpulse, and FIG. 4B illustrates generally a spectrogram of thetime-domain waveform of FIG. 4A.

FIG. 5 illustrates generally an illustrative example of a comparisonbetween a direct-absorption spectroscopy technique using a broadbandchirped-pulse frequency comb (CP-FC) spectrometer, and a segmented CP-FTapproach.

FIGS. 6A through 6B illustrate generally an illustrative example of anexperimentally-obtained spectrum for vinyl cyanide from 260 GHz to 290GHz, using a segmented CP-FT approach, as compared to a simulatedspectrum.

FIGS. 7A through 7B illustrate generally an illustrative example of anexperimentally-obtained spectrum for methanol from 0.790 THz to 0.850THz, using a segmented CP-FT approach, as compared to a simulatedspectrum.

FIG. 8 illustrates generally illustrative examples ofexperimentally-obtained spectra using a segmented CP-FT approach, andincluding a double-resonance measurement technique.

FIG. 9 illustrates generally a technique, such as a method for obtainingan emission from a sample in response to excitation using a specifiedrange of excitation frequencies.

FIG. 10 illustrates generally illustrative examples ofexperimentally-obtained spectra using a single-channel AWGimplementation as compared to a dual-channel AWG implementation.

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

DETAILED DESCRIPTION

FIG. 1 illustrates generally an example of a system 100, such as can beused for generating a chirped-pulse excitation signal, or for obtainingemission from a sample in response to such excitation. In an example,the system 100 can include at least one processor circuit 106, such ascoupled to a memory circuit 108 (or one or more other storage circuitsor devices). The processor circuit 106 can be coupled to a high-speedDAC 102, such as include one or more output channels, such as a firstchannel 124A, or a second channel 124B.

In an example, the system 100 can include a frequency reference 104,such as configured to provide a stabilized reference frequency for useby one or more other portions of the system 100. In an example, thesystem 100 can include a first mixer 122A, such as configured upconverta chirped waveform using a specified upconversion LO frequency. Thechirped waveform can be provided by the high-speed DAC 102. Theupconversion LO frequency can be provided by an oscillator 112. In anexample, the first mixer 122A can be configured to provide anupconverted chirped waveform to a frequency multiplier 114A, and thefrequency multiplier 114A can output a chirped excitation signal withina specified frequency range for use in exciting a sample 116.

In an example, the system 100 can include a second mixer 118, such asconfigured to receive an emission from the sample 116, and todownconvert the received emission using a signal based on a specifieddownconversion LO signal. The downconverted emission signal can beprovided to input of an ADC 120, within the available bandwidth of theADC 120. The processor circuit 106 can be configured to estimate aspectrum of the emission signal using information obtained via the ADC120.

The specified downconversion LO signal can be provided by the high-speedDAC 102, using one or more of the first or second channels 124A or 124B.The specified downconversion LO signal can be upconverted using thefirst mixer 122A or a third mixer 122B. The upconverted LO signal canthen be frequency multiplied using the first frequency multiplier 114Aor a second frequency multiplier 114B, such as to provide the signalbased on the specified downconversion LO signal for use by the secondmixer 118. In this manner, the high-speed DAC 102 can generate arapidly-tunable LO signal that can then be routed through the mixer andmultiplier chain for use in downconverting the received emissions fromthe sample 116 using the second mixer 118. In one approach, the mixerand multiplier chain can be independent of the chain used for thechirped-pulse excitation signal, such as shown by the dotted lines inFIG. 1, and in the illustrative example of FIG. 2A.

In another approach, the mixer and multiplier used for thedownconversion LO signal can be the same as used for excitation of thesample 116, and the path followed by the chirped-excitation pulse can bethe same as the path followed by the downconversion LO signal (e.g., thechirped excitation signal delivered to the sample 116 can spatiallyoverlap with the LO signal to be used in downconverting the receivedemissions), such as if the dotted portions are omitted from FIG. 1, andas shown in the illustrative example of FIG. 2B.

FIG. 2A illustrates generally an illustrative example of a mm-wavespectrometer system 200A, such as operable in a frequency range fromabout 260 GHz to about 290 GHz, and including a sub-harmonic mixer 218Ahaving two input ports. In this illustrative example, the system 200Acan include an AWG 202 having two channels, such as including a firstchannel 224A that can be used to generate the chirped excitation pulse(e.g., upconverted, frequency multiplied, or amplified before passingthrough a gaseous sample in a chamber 216), and second channel 224B toprovide a specified downconversion LO frequency for use indownconverting the molecular free induction decay (FID) signal to alower frequency range where it can be digitized by a digitizer 220.

In the example of FIG. 2A, similar to the example of FIG. 1, a firstmixer 222A be used to upconvert a chirped waveform prior tomultiplication by a 24× multiplier chain 214A. One or more amplifiers,filters, or isolators (e.g., one or more circulators) can be used tofurther condition the chirped waveform, such as shown in the examples ofFIGS. 2A and 2B. In the examples of FIGS. 2A and 2B, an 8.8 GHzphase-locked dielectric resonator oscillator (PDRO) 212 can provide anupconversion LO frequency to one or more of the first mixer 222A or asecond mixer 222B.

In the example of FIG. 2A, the second mixer 222B can upconvert thereceived downconversion LO frequency, such as using the PDRO 212, andthe resulting upconverted signal can then be frequency multiplied usinga second frequency multiplier chain 214B, such as including a 12×multiplication factor. The sub-harmonic mixer 218 can provide anadditional inherent 2× multiplication factor, such as for downconvertingreceived emission from the sample within the chamber 216 into abandwidth range suitable for the digitizer 220, thus effectively thedownconversion LO pathway can provide the same multiplication factor asthe 24× multiplier chain 214A.

In an illustrative example, such as to provide theexperimentally-obtained information for the examples herein, the AWG 202can use a sampling rate of about 12 Gs/s (e.g., about 83 picosecond (ps)time resolution). In an example, a single-channel AWG can be used in thesub-harmonic mixer design of FIG. 2A, such as by adding a single-poledouble-throw (SPDT) switch to the first channel 224A and directing thechirped pulse and LO signals to their respective multiplier chains, orusing a configuration shown in the illustrative example of FIG. 2C.

In the dual-AWG-channel approach shown in the example of FIG. 2A, thedownconversion LO signal is generally output prior to the chirped-pulsecreation. The present inventors have recognized, among other things,that an advantage of this arrangement is that a chirped excitation pulsegenerated using the first channel that is transmitted through the samplechamber 216 can be monitored (e.g., with removal of the high-gainpreamplifier), such as to assess the quality of the chirped excitationpulse, but at the cost of using the dual-channel approach as compared tosingle-channel arrangements discussed elsewhere herein.

FIG. 2B illustrates generally an illustrative example of a mm-wavespectrometer system 200B, such as including a fundamental-mode mixer218B having two input ports. In an example, the fundamental-mode mixer218B can be used for mm-wave/THz downconversion to a frequency rangesuitable for the digitizer 220. Generally, the fundamental-mode mixer218B uses an LO signal path and an excitation signal path that spatiallyoverlap. The physics of chirped-pulse Fourier transform spectroscopyprovides that the molecular emission follows the same path as the lightfrom the excitation source. Therefore, the segmented CP-FT measurementcan be made by using a single AWG channel that has the chirped pulseduring a first duration followed by an LO frequency during a secondduration, such as in a single waveform. Such a waveform can betransmitted through the sample and the molecules will emit their freeinduction decay following chirped-pulse excitation. Such FID emissioncan then mix with (e.g., beat against) the LO frequency beingtransmitted through the sample at the fundamental-mode mixer 218B.

FIG. 2C illustrates generally an illustrative example of a mm-wavespectrometer system 200C, such as including a sub-harmonic mixer 218Aand a switch 230, such as to provide a spectrometer including asingle-channel arbitrary waveform generator (AWG) implementation. Incontrast to the configuration shown in FIG. 2A, the configuration ofFIG. 2C can use a single (e.g., shared) AWG channel 224A to provide botha downconversion LO signal and a chirped excitation pulse.

For example, a first mixer 222A can be coupled to an oscillator (e.g., aPDRO 212 similarly to the examples of FIGS. 2A and 2B), such as forupconversion of both the LO signal generated by the AWG 202, and achirped excitation pulse generated by the AWG 202. The present inventorshave recognized, among other things, that a complexity and cost of thesystem 200C can be heavily influenced by the number of high-speed AWGchannels included in the system. The single AWG channel 224A included inthe example of FIG. 2C can help to reduce system complexity andcomponent count, along with a corresponding reduction in cost, withoutcompromising measurement quality (as shown in the illustrative exampleof FIG. 10).

The system 200C of FIG. 2C can be operated in a manner where the AWG 202generates a single waveform that includes the chirped excitation pulsefollowed by a specified single frequency sign wave for use as thedownconversion LO signal. For example, such a single frequency sine waveoutput can be generated immediately or otherwise closely following thechirped excitation pulse. The output of the frequency upconversionprovided at the output of the first mixer 222A can be power divided intotwo signal lines, including a first path for the chirped excitationpulse, including a 24X multiplier chain 214A and the sample chamber 216,and a second path including a 12X multiplier chain 214B to provide thedownconversion LO signal to the sub-harmonic mixer 218A. In an example,a switch 230, such as a Transistor-Transistor-Logic (TTL) level drivenmicrowave PIN-diode switch, can be included in the first path to allow achirped excitation pulse to pass to the sample chamber 216, andsuppressing or inhibiting passage of the downconversion LO signal. Inthis manner, the configuration shown in FIG. 2C allows thedownconversion LO signal to bypass the sample chamber. In an example,the trigger or control signal for the switch can be obtained using amarker channel output of the AWG 202.

Generally, excitation and detection are time-separated events in CP-FTspectroscopy, as discussed further elsewhere herein. Accordingly, thereceive signal path does not need to be in its proper operating stateuntil the end of a respective chirped excitation pulse. In addition,because the chirped-pulse must pass through the sample cell (e.g.,chamber 216), it will generally be time-delayed relative to the LOpulse. This time delay can provide sufficient duration for the LO pulseto be generated and have all transient responses sufficiently dissipatebefore the receive signal path starts to downconvert the molecular freeinduction decay (FID). In an illustrative example ofexperimentally-obtained system performance, the main delay, or deadtime, is the recovery of the high gain, low noise IF amplifier 250 fromsaturation once the chirped excitation pulse ends. Generally, there isno way to protect the receive signal path from the excitation pulse formm-wave measurements because an extremely high-speed broadband switchwould be needed, and placing a switch between the sub-harmonic mixer218A and IF amplifier 250 would add the switch conversion loss to theoverall receiver noise figure while producing transients of its own.Such dead time has been measured to be about 200 ns inexperimentally-derived measurements, and generally appears to be longerthan any transient settling time of the sub-harmonic mixer 218A.

As shown in FIG. 10, little if any visible difference exists betweenmeasurements obtained with a single-channel configuration (e.g., asshown in FIG. 2C), as compared with a dual channel implementation (e.g.,as shown in FIG. 2A). The illustrative example of FIG. 2C includes a 100GS/s digitizer 221. Such a digitizer can be used in relation to otherexamples, such as shown in FIG. 1, or FIG. 2A or 2B.

FIG. 3A illustrates generally an illustrative example of a spectrogram300 that can represent respective DAC channel outputs, such ascorresponding to a dual-channel example, and FIG. 3B illustratesgenerally a detailed view of a portion of FIG. 3A. A feature of CP-FTcoherent spectroscopy techniques is that the sample excitation and FIDdownconversion process are separated in time. In FIG. 3B, a firstchirped waveform 310A and a specified first LO downconversion frequency320A can be provided during at least a portion of a first duration 330A(e.g., a first “segment”). In a second “segment,” a second chirpedwaveform 310B, and a specified second LO downconversion frequency 320Bcan be provided during at least a portion of a second duration 330B.

The full available measurement bandwidth of the spectrum (e.g.,determined by the bandwidth of the mm-wave/THz multiplier chains or byother system elements) can be segmented for the measurement. In anexample, a single pass through the full spectrometer bandwidth caninclude a series of chirped excitation pulses and a respectivedownconversion LO frequencies. For example, the LO and chirp frequenciescan be increased for each successive segment. For signal averaging inthe time domain, the relative phase of the chirped pulse and LO for eachsegment are reproduced in each pass through the full spectrum. The useof an AWG provides phase reproducibility between sweeps, and provides acapability to switch the LO frequency (with phase lock) on time scalesthat are short compared to the molecular FID transients.

In an example, in a CP-FT segment, one channel of the arbitrary waveformgenerator can be used to provide a short-duration linear frequency sweep(e.g., a chirped waveform), which can then be upconverted such using amixer coupled to a PDRO, and then multiplied after upconversion. Becausethe chirped waveform only includes a single frequency at any given time,the duration of the pulse is unchanged in the multiplication step butthe bandwidth increases. This pulse interacts with a molecular gassample, and can induce a macroscopic polarization when a rotationaltransition within the bandwidth of the excitation pulse is crossed.After the chirped pulse, the second AWG channel provides asingle-frequency LO pulse that is offset from the sweep range by a smallamount (e.g., to limit low-frequency spurious outputs or the effect of1/f noise). The molecular sample can continue to emit radiation at thefrequencies of the molecular transitions in the sweep range, and suchradiation can be downconverted with the LO pulse and digitized. Eachsegment is Fourier transformed, and the segments are pieced together tocreate a broad-bandwidth spectrum.

In addition to overcoming digitizer bandwidth limitations, the segmentedCP-FT approach can be more computationally efficient than determining aspectrum estimate for an entirety of the bandwidth being measured. Forexample, the number of computations included in a Cooley-Tukey fastFourier transform technique can scale as N log₂ N, where N can representthe number of data points in the trace. Therefore, a segmented CP-FTspectrum can include fewer computations to Fourier transform a broadbandwidth spectrum than Fourier transforming a single spectrum coveringthe same total bandwidth.

FIG. 4A illustrates generally an illustrative example of a measuredtime-domain waveform including a chirped pulse followed by asingle-frequency LO pulse, and FIG. 4B illustrates generally aspectrogram of the time-domain waveform of FIG. 4A.

In the illustrative examples of FIGS. 4A and 4B, the time domain andspectrogram of a pulse includes a chirped excitation pulse 410 (e.g.,about 125 nanoseconds (ns) duration) that can be followed by a singlefrequency LO pulse 420 (e.g., about 1.875 μs duration). The time-domainpulse of the illustrative example of FIG. 4A was obtained using thesystem 200A of the example of FIG. 2A. In the illustrative example ofFIG. 4B, the spectrogram, which shows only about the first 1 μs of thewaveform spectrogram, clearly shows that the AWG-generated waveformincludes a rapid chirped excitation pulse 410 followed by a singlefrequency output for use as the LO pulse 420.

For example, a single AWG channel can be used because the excitationperiod and detection period are separated in time. Therefore, in anexample, the AWG waveform in a respective measurement segment caninclude the chirped pulse 410, such as followed immediately by thesingle frequency LO waveform 420.

Immediate switching between chirped excitation and single-frequency LOoutput modes is supported by the AWG. In a fundamental-mode mixerexample, such as shown in FIG. 2B, the LO output is directed through thesample. In a sub-harmonic mixer example, a SPDT switch external to theAWG can be used to direct the chirped excitation pulse and the LO pulsefor FID downconversion to respective multiplier chains, or a SPST switchexternal to the AWG can be used to isolate the multiplier chain andsample cell during generation or the LO pulse for downconversion.

FIG. 5 illustrates generally an illustrative example 500 of a comparisonbetween a transmission spectrum 550 experimentally obtained viadirect-absorption spectroscopy technique using a broadband chirped-pulsefrequency comb (CP-FC) spectrometer, and an emission spectrum 560experimentally obtained via segmented CP-FT approach. Unlike absorptionspectroscopy techniques, in a CP-FT approach, background calibration canbe performed without a reference cell or sample evacuation.

Because the AWG output is generally not as pure in frequency as aphase-locked oscillator or synthesizer, spurious outputs can occur. Inan example, to subtract these effects, a background trace can becollected before the chirped excitation pulse is delivered. For example,the molecular FID signals only occur after excitation by a chirpedpulse, so the background trace can be collected in the presence of thegas sample, such as without requiring a reference cell. Inexperimentally-obtained samples, up to near 550 GHz, such backgroundcalibration was not necessary.

In one approach, a chirped-pulse frequency comb (CP-FC) technique canoffer similar measurement speed, detection bandwidth, and frequencyaccuracy in comparison to the segmented CP-FT technique. However, incontrast to CP-FT, CP-FC spectroscopy is an absorption technique, somolecular signals are measured as depletions in the background pulse. Asdiscussed above, for both microwave and infrared techniques, molecularFID emissions, which can be measured against much lower backgrounds, aregenerally more sensitive techniques for molecular spectroscopy.

In the illustrative example of FIG. 5, a rotational spectrum of vinylcyanide (acrylonitrile, CH₂═CHCN) is shown. The sample pressure was 5milliTorr (mTorr) with a 4 meter (m) path length. The spectra shown inFIG. 5 shows a portion of the full available 260-290 GHz measurementrange. For equal measurement times (e.g., about 2 ms for the CP-FT andabout 1 ms for the CP-FC—where the CP-FC measurement also included anadditional 1 ms measurement of the background), the sensitivity in CP-FTcan be about a factor of 10 higher (e.g., a signal-to-noise ratio (S/N)can be about 230:1 for the segmented CP-FT technique as compared toabout 15:1 for the CP-FC technique).

FIGS. 6A through 6B illustrate generally an illustrative example of anexperimentally-obtained spectrum for vinyl cyanide from 260 GHz to 290GHz, using a segmented CP-FT approach, as compared to a simulatedspectrum. In FIGS. 6A and 6B, a measured spectrum 610A can be comparedto a simulated spectrum 610B, the simulated spectrum 610B provided usinginformation from the NASA Jet Propulsion Laboratory (JPL) database. Theexperimentally-obtained spectrum 610A was acquired using segmentscorresponding to chirped excitation pulses having about 480 MHzbandwidth, and the downconversion LO was offset by 240 MHz from theexcitation bandwidth. The chirped pulse duration was 125 ns and the peakpower of the mm-wave excitation pulse was 1 milliwatt (mW). The FID wasdigitized starting at 40 ns after the end of the excitation pulse (e.g.,to allow the receiver to recover from saturation by the excitationpulse) and the FID was collected over a duration of 1.835 μs. The totaltime for each segment was 2 μs. There were 75 segments in the fullspectrum for a total spectrum acquisition time of 150 μs. The samplepressure was about 5 mTorr and the path length was 4 m.

FIGS. 7A through 7B illustrate generally an illustrative example of anexperimentally-obtained spectrum 710A for methanol from 0.790 THz to0.850 THz, using a segmented CP-FT approach, as compared to a simulatedspectrum.

The frequency range of the example of FIGS. 7A and 7B was made availableby adding an additional frequency tripler to the LO and chirped pulseexcitation multiplier chains, using a configuration similar to theillustrative example of FIG. 2A. In this illustrative methanolmeasurement example, the chirped waveform bandwidth corresponding toeach segment was also about 500 MHz. However, the faster dephasing(e.g., from Doppler broadening) at higher frequency (and lower samplemass) allowed reduction of the segment time duration to 500 ns.Approximately 60 GHz of spectrum was acquired using 120 segments and atotal measurement time of 60 μs. The sample pressure was 40 mTorr andthe path length about 10 m. In the illustrative example of FIG. 7B, aportion of the spectrum 710A is compared to a simulated spectrum 710Bobtained using information from the NASA JPL catalog.

FIG. 8 illustrates generally an illustrative example 800 ofexperimentally-obtained spectra using a segmented CP-FT approach, andincluding a double-resonance measurement technique. As a coherentspectroscopy, the CP-FT method permits double-resonance measurementsthat can produce high signal modulation. Double-resonance spectroscopycan be a useful technique for identifying an unknown spectrum by showingthe “connectivity” of the rotational spectroscopy transitions. Suchconnectivity occurs when transitions share a common quantum state. Suchtechniques can be especially useful when the measured spectrum comesfrom a complex mixture of different species.

In an example, a double-resonance measurement can include using threewaveforms that can be separate in time and, for example, generated by asingle AWG channel. In an illustrative example, two excitation pulsesthat interact with the molecular sample can be generated using one AWGchannel, and the LO can be generated using a second AWG channel, such asin a sub-harmonic mixer design (e.g., as shown in the illustrativeexample of FIG. 2A).

In an example, the excitation pulses include first a chirped pulse thatexcites a segment of the rotational spectrum and coherently excited thesample, and a second pulse following the chirped-pulse, the second pulsecomprising a single fixed frequency excitation. In an example, thefrequency of the second pulse is selected to be coincident with atransition in the spectrum. The effect of this frequency-selective pulsecan be to destroy the coherence for any transition that shares a quantumstate with the selected “pumped” transition.

This causes a detectable reduction in the intensity of any transitionsexcited by the chirped pulse that are in “double-resonance.” Theillustrative example of FIG. 8 includes a double-resonance measurementfor the vinyl cyanide (acrylonitrile) spectrum of FIGS. 6A and 6B. Inthis illustrative example, a chirped excitation pulse can polarize thesample in the frequency segment of 285.360-285.840 GHz. As discussed inrelation to FIG. 6A, a first spectrum can be experimentally obtained,such as including a rotational transition 820A that has been previouslyanalyzed and “assigned” as 30_(4.26)-29_(4.25) (e.g., a standardasymmetric top quantum level designation of molecular spectroscopy:J_(Ka,Kc).

The frequency for this transition is 285.5926 GHz. The selectiveexcitation pulse included a frequency of 275.9974 GHz and is resonantwith the previously assigned 29_(4.25)-28_(4.24) transition. Becausethese two transitions share a quantum level (29_(4.25)), the selectiveexcitation pulse reduces the signal intensity of the monitored30_(4.26)-29_(4.25) transition (by about 50%), at 820B, withoutaffecting nearby transitions not in double-resonance. The performance ofthe selective excitation double-resonance scheme is highlighted bydisplaying a difference spectrum 830 at the bottom of the figure,showing a prominent dip 840 corresponding to the 30_(4.26)-29_(4.25)transition 820A at 285.5926 GHz.

In an example, double resonance spectroscopy can be used such as tocreate a 2-D spectrum showing all the pairs of transitions that share aquantum state in the spectrum, such as similar to techniques that can beused for 2-D nuclear magnetic resonance (NMR) spectroscopy. Because ofthe large number of individual measurements performed in the segmentedCP-FT technique, a new double resonance measurement can be performed inevery segment, allowing for faster determination of the 2-D spectrum,and allowing for more rapid interpretation of complex unknown spectra.

FIG. 9 illustrates generally a technique 900, such as a method forobtaining an emission from a sample in response to excitation using aspecified range of excitation frequencies. The technique 900 can includeusing one or more portions of the apparatus of the examples of FIG. 1,2A, or 2B.

At 902, a specified chirped waveform and a specified downconversion LOfrequency can be generated, such as using a high-speed DAC (e.g., an AWGor one or more other circuits or systems). At 904, the chirped waveformcan be upconverted, such as via mixing the chirped waveform with aspecified upconversion LO frequency. At 906, the upconverted chirpedwaveform can be frequency multiplied to provide a chirped excitationsignal for exciting the sample.

At 908, an emission can be received from the sample, the emissionelicited at least in part by the chirped excitation signal. At 910, thereceived emission can be downconverted, such as by mixing the receivedemission with a signal based on the specified downconversion LO signalto provide a downconverted emission signal within the bandwidth of anADC converter. In an example, the technique can include a segmentedCP-FT technique, such as including generating a first specified chirpedwaveform during a first duration, and a second chirped waveform during asecond duration, the first and second chirped waveforms includingrespective bandwidths specified based at least in part on the bandwidthof the ADC and including a total bandwidth corresponding to thespecified range of excitation frequencies.

FIG. 10 illustrates generally illustrative examples ofexperimentally-obtained spectra using a single-channel AWGimplementation as compared to a dual-channel AWG implementation. Thesingle-channel AWG implementation corresponds to the system shown in theillustrative example of FIG. 2C, which was used to provide a firstspectrum 1050A, and the dual-channel AWG implementation corresponds tothe system shown in the illustrative example of FIG. 2A, which was usedto provide the second spectrum 1050B, but with a 100 GS/s digitizerprovided as the analog-to-digital converter (ADC). The molecular sampleused for experimentally obtaining the first and second spectra 1050A and1050B was ethyl cyanide at 0.5 mT pressure. As shown in FIG. 10, nosignificant difference in the obtained spectra 1050A and 1050B isvisible.

In an illustrative example, the CP-FT techniques and apparatus describedherein can be used to obtain high-sensitivity spectra across largebandwidths (e.g., up to about 100 GHz or more), with an acquisition timethat can be about 1 ms or less. In an illustrative example, a relativelylow digitizer detection bandwidth (e.g., less than about 1 GHz) can beused. Generally, using the CP-FT techniques described herein, thereceived signals can be coherently detected in the time domain, sosignal averaging (or determination of one or more other centraltendencies) can be performed, such as to enhance sensitivity as desiredfor a specific application.

Because the molecular signals can be digitized in the radio-frequencyregion of frequencies (rather than the mm-wave range), and the localoscillator sources are referenced to a high-precision (e.g., rubidium)frequency standard, the absolute frequency accuracy of the segmentedCP-FT technique can be extremely high, such as limited only by theuncertainty in the molecular line width, and no calibrant is necessary.In contrast to CP-FT approaches, some other approaches requirecalibration such as using spectral lines of known frequency, or use ofetalons, such as to correct for a nonlinearity of a frequency sweepingsource on every scan.

Various Notes & Examples

Example 1 can include subject matter (such as an apparatus, a method, ameans for performing acts, or a machine readable medium includinginstructions that, when performed by the machine, that can cause themachine to perform acts), such as can include a method for obtaining anemission from a sample in response to excitation using a specified rangeof excitation frequencies, the method comprising generating a specifiedchirped waveform and a specified downconversion local oscillator (LO)frequency using a digital-to-analog converter (DAC), upconverting thechirped waveform via mixing the chirped waveform with a specifiedupconversion LO frequency, frequency multiplying the upconverted chirpedwaveform to provide a chirped excitation signal for exciting the sample,receiving an emission from the sample, the emission elicited at least inpart by the chirped excitation signal, downconverting the receivedemission via mixing the received emission with a signal based on thespecified downconversion LO signal to provide a downconverted emissionsignal within the bandwidth of an analog-to-digital converter (ADC). InExample 1, the generating the specified chirped waveform can includegenerating a first chirped waveform during a first duration, and asecond chirped waveform during a second duration, the first and secondchirped waveforms including respective bandwidths specified based atleast in part on the bandwidth of the ADC and including a totalbandwidth corresponding to the specified range of excitationfrequencies.

Example 2 can include, or can optionally be combined with the subjectmatter of Example 1, to optionally include estimating a first spectrumcorresponding to the specified range of excitation frequencies,including using information corresponding to respective emissionsobtained during at least a portion of the first and second durations.

Example 3 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 1 or 2 to optionallyinclude generating a specified fixed frequency to excite the sampleafter generating the chirped excitation signal, and estimating a secondspectrum using information corresponding to an emission obtained fromthe sample in response to the chirped excitation signal and in responseto the specified fixed frequency.

Example 4 can include, or can optionally be combined with the subjectmatter Example 3 to optionally include a specified fixed frequency thatmodulates an energy level transition of the sample after the sample iscoherently excited using the chirped excitation signal.

Example 5 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 3 or 4 to optionallyinclude D determining a relative indication of information using theestimated first and second spectra.

Example 6 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 1 through 5 to optionallyinclude receiving respective first and second downconverted receivedemissions obtained via repeating the chirped excitation of the sample,and determining a central tendency of information obtained from thefirst and second downconverted received emissions.

Example 7 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 1 through 6 to optionallyinclude receiving an emission comprising a free-induction decay emissionfrom a sample comprising a gaseous species.

Example 8 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 1 through 7 to optionallyinclude generating the specified chirped waveform includes using a firstchannel of an arbitrary waveform generator (AWG), the generating thespecified downconversion LO frequency including using a second channelof an AWG, and the downconverting the emission signal including using asub-harmonic mixer comprising a first port configured to receive thesignal based on the specified downconversion LO frequency, a second portconfigured to receive the emission; and a third port configured toprovide the downconverted emission signal within the bandwidth of theADC.

Example 9 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 1 through 7 to optionallyinclude E upconverting the specified downconversion LO frequency andfrequency multiplying the upconverted output to provide the signal basedon the downconversion LO frequency, the downconverting the emissionincluding using a fundamental mode mixer comprising a single input portconfigured to receive the signal based on the downconversion LOfrequency and the emission, and a second port configured to provide thedownconverted emission signal, the signal based on the LO frequencypropagating via the same spatial path as the chirped excitation signaland the elicited emission.

Example 10 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 1 through 9 to optionallyinclude a frequency reference comprising a precision oscillatorconfigured to provide a reference frequency based at least in part on anatomic or molecular energy level transition.

Example 11 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 1-10 to include, subjectmatter (such as an apparatus, a method, a means for performing acts, ora machine readable medium including instructions that, when performed bythe machine, that can cause the machine to perform acts), such as caninclude a digital-to-analog converter (DAC) coupled to a frequencyreference and configured to provide a specified chirped waveform and aspecified downconversion local oscillator (LO) frequency, a phase-lockedoscillator coupled to the frequency reference and configured to providea specified upconversion LO frequency, a first mixer configured toreceive the specified chirp waveform from the DAC and the specifiedupconversion LO frequency from the phase-locked oscillator, andconfigured to provide an upconverted chirped waveform, a frequencymultiplier configured to receive the upconverted chirped waveform andconfigured to provide a chirped excitation signal for exciting a sample,and a second mixer configured to receive an emission from the sample,the emission elicited at least in part by the chirped excitation signal,the second mixer configured to receive the specified downconversion LOfrequency from the DAC, and configured to provide an output signalwithin the bandwidth of an analog-to-digital converter. In Example 11,the DAC can be configured to generate a first specified chirped waveformduring a first duration, and a second chirped waveform during a secondduration, the first and second chirped waveforms including respectivebandwidths specified based at least in part on the bandwidth of the ADCand including a total bandwidth corresponding to the specified range ofexcitation frequencies.

Example 12 can include, or can optionally be combined with the subjectmatter of Example 11, to optionally include an ADC coupled to the secondmixer, and a processor coupled to the ADC, the processor configured toestimate a first spectrum corresponding to the specified range ofexcitation frequencies, including using information corresponding torespective emissions obtained during at least a portion of the first andsecond durations.

Example 13 can include, or can optionally be combined with the subjectmatter of Example 12, to optionally include a DAC configured to generatea specified fixed frequency to excite the sample after generating thechirped excitation signal, and a processor configured to estimate asecond spectrum using information corresponding to an emission obtainedfrom the sample in response to the chirped excitation signal and inresponse to the specified fixed frequency.

Example 14 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 11 through 13 to optionallyinclude an emission comprising a free-induction decay emission from asample comprising a gaseous species.

Example 15 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 11 through 14 to optionallyinclude an arbitrary waveform generator (AWG) including the DAC, the AWGconfigured to generate the specified chirped waveform using a firstchannel of the AWG, and configured to generate the specifieddownconversion LO frequency using the second channel of the AWG, and asecond mixer comprising a sub-harmonic mixer including a first portconfigured to receive the signal based on the specified downconversionLO frequency, a second port configured to receive the emission; and athird port configured to provide the downconverted emission signalwithin the bandwidth of the ADC.

Example 16 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 11 through 14 to optionallyinclude a third mixer configured to upconvert the specifieddownconversion LO frequency to provide an upconverted output, and asecond frequency multiplier configured to frequency multiply theupconverted output to provide the signal based on the downconversion LOfrequency, the second mixer comprising a fundamental mode mixerincluding a single input port configured to receive the signal based onthe downconversion LO frequency and the emission, and a second portconfigured to provide the downconverted emission signal, and the signalbased on the LO frequency propagating via the same spatial path as thechirped excitation signal and the elicited emission.

Example 17 can include, or can optionally be combined with the subjectmatter of one or any combination of Examples 11 through 16 to optionallyinclude a frequency reference including a precision oscillatorconfigured to provide a reference frequency derived at least in partfrom an atomic or molecular energy level transition.

Example 18 can include, or can optionally be combined with any portionor combination of any portions of any one or more of Examples 1-17 toinclude, subject matter that can include means for performing any one ormore of the functions of Examples 1-20, or a machine-readable mediumincluding instructions that, when performed by a machine, cause themachine to perform any one or more of the functions of Examples 1-20.

Each of these non-limiting examples can stand on its own, or can becombined in various permutations or combinations with one or more of theother examples.

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples.” Such examples can include elements in addition tothose shown or described. However, the present inventors alsocontemplate examples in which only those elements shown or described areprovided. Moreover, the present inventors also contemplate examplesusing any combination or permutation of those elements shown ordescribed (or one or more aspects thereof), either with respect to aparticular example (or one or more aspects thereof), or with respect toother examples (or one or more aspects thereof) shown or describedherein.

In the event of inconsistent usages between this document and anydocuments so incorporated by reference, the usage in this documentcontrols.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In this document, the terms “including” and “inwhich” are used as the plain-English equivalents of the respective terms“comprising” and “wherein.” Also, in the following claims, the terms“including” and “comprising” are open-ended, that is, a system, device,article, composition, formulation, or process that includes elements inaddition to those listed after such a term in a claim are still deemedto fall within the scope of that claim. Moreover, in the followingclaims, the terms “first,” “second,” and “third,” etc. are used merelyas labels, and are not intended to impose numerical requirements ontheir objects.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods can include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code can include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, in an example, the code can be tangiblystored on one or more volatile, non-transitory, or non-volatile tangiblecomputer-readable media, such as during execution or at other times.Examples of these tangible computer-readable media can include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact disks and digital video disks), magnetic cassettes,memory cards or sticks, random access memories (RAMs), read onlymemories (ROMs), and the like.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription as examples or embodiments, with each claim standing on itsown as a separate embodiment, and it is contemplated that suchembodiments can be combined with each other in various combinations orpermutations. The scope of the invention should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

The claimed invention is:
 1. A method for obtaining an emission from asample in response to excitation using a specified range of excitationfrequencies, the method comprising: generating a specified chirpedwaveform and a specified downconversion local oscillator (LO) frequencyusing a digital-to-analog converter (DAC); upconverting the chirpedwaveform via mixing the chirped waveform with a specified upconversionLO frequency; frequency multiplying the upconverted chirped waveform toprovide a chirped excitation signal for exciting the sample; receivingan emission from the sample, the emission elicited at least in part bythe chirped excitation signal; and downconverting the received emissionvia mixing the received emission with a signal based on the specifieddownconversion LO signal to provide a downconverted emission signalwithin the bandwidth of an analog-to-digital converter (ADC); whereinthe generating the specified chirped waveform includes generating afirst chirped waveform during a first duration, and a second chirpedwaveform during a second duration, the first and second chirpedwaveforms including respective bandwidths specified based at least inpart on the bandwidth of the ADC and including a total bandwidthcorresponding to the specified range of excitation frequencies.